Lifting hooks with various safety arrangements are well-known in the art, the two predominant ones, being illustrated in the appended drawings, FIGS. 1 and 2.
FIG. 1 shows a conventional lifting hook with an arcuate hook body 101 which at its upper part is provided with a suspension means in the form a transversal, detachable pin 102 to be coupled to the end link of a hoisting chain (not shown) and a pivotable closure arm 103. The latter is spring-loaded to a closing position (as shown), where it closes the hook opening 104 and holds a lifting gear represented here by a link 105 (coupled to a load, not shown) securely in place. The arm 103 can be swung against the spring means into an inward position where it keeps the hook opening 104 free and enables insertion of a link 105 or similar into the inside of the hook. As is well-known in the art, the pin 102 can be replaced by a closed eye or a swivel member which is attachable to a hoisting chain, a rope, a wire or a hoisting strap. According to some proposals, see e.g. the Norwegian laid-open print 20013434 (Ekeskog et al) and the Japanese published patent application JP2001253679), a further closure arm is disposed in the hook opening, one being pivotable inwardly and the other being pivotable outwardly in relation to the tip of the hook body. However, such an arrangement is not very practical, since the operator has to manipulate both closure arms manually, which is difficult and somewhat risky.
FIG. 2 illustrates a self-locking lifting hook of the kind defined in the opening paragraph, including an arcuate hook body 111 with a fork-like upper portion 112, where a two-arm lever 113 is pivotally journalled on a transverse pin 114 extending between the two shanks of the upper portion 112.
The lever 113 comprises an upper arm 115, provided with a suspension pin 116 (or some other suspension member), and a lower closure arm 117, which in a loaded situation keeps the hook opening 120 closed so as to retain a link 118 securely in the hook. Often, there is a spring loaded locking mechanism in the upper part 112 of the hook body, which can hold the closure arm either in the closed position (as shown) or in a fully open position.
In case of the conventional lifting hook according to FIG. 1, the lifting gear represented by the link 105 can in a non-loaded situation unintentionally swing to a position above the hook tip, see FIG. 1, where it when loaded or by its own weight can force the closure arm 103 to open, resulting in dropped lifting gear/load.
The traditional self locking hook according to FIG. 2 withstands the above described risk due to its design having a strong closure arm 117 pivotable outwardly. The self locking hook type remains due to its design always in a closed position as long as it is loaded. The locking mechanism is intended to secure the closure arm 117 in a closed position also in non-loaded situations. However the locking mechanism can unintentionally become released due to a direct hit on the locking mechanism or by momentum of inertia if the hook collides with a hard structure. Alternatively, it may happen that a possible locking mechanism (in the embodiment of FIG. 2) becomes inoperable e.g. due to wear, breakage of a spring or for some other reason, resulting in risk of dropping lifting gear/load.